While we have known for centuries that facial expressions can reveal what people are thinking and feeling, it is only recently that the face has been studied scientifically for what it can tell us about internal states, social behavior, and psychopathology. Today's widely available, sophisticated measuring systems have allowed us to conduct a wealth of new research on facial behavior that has contributed enormously to our understanding of the relationship between facial expression and human psychology. The chapters in this volume present the state-of-the-art in this research.